Room temperature-vulcanizing silicone rubber has enjoyed increasingly wider use in dentistry as a molding medium in the preparation of artificial dentures. The material is used as a paste into which a curing agent is mixed before use. It then changes from a pasty consistency to a rubber-elastic state upon curing at body temperature. Because of its elasticity, the cured silicone can be removed from the mouth easily and can then be used for casting a working model, such as with plaster of paris, from which the permanent dental piece is made.
Silicone rubber compositions offer several advantages over materials formerly used as a dental replica material, e.g., plaster, alginate, etc. As compared with plaster, silicone rubber is more elastic, has better resistance to breakage and offers good release from the jaw. As compared with alginates, silicone rubber is not sensitive to loss of water (which may cause shrinkage) and possesses better molding accuracy. Silicones also provide good dimensional stability which is retained even on prolonged storage in the air (For reference, see "Chemistry and Technology of Silicones", Walter Noll, Academic Press, New York, 1968, p. 623.)
Elastomeric compositions have been utilized in two main areas of dental impression work -- one, crown and bridge work and, two, preparation of full dentures. Crown and bridge work requires a custom tray which is usually prepared by first making an impression with alginate of the area of the mouth to be repaired. A cast of this impression is then made by pouring dental stone into the hardened alginate and curing it. The cast is then wrapped with asbestos and a form is built up around the wrapped casting using methyl methacrylate or some other suitable polymeric material (e.g., polyethylene, polyvinyl chloride, polyepoxide, etc.). After the methyl methacrylate has been cured, it is removed from around the dental stone casting and used as the custom tray for the silicone rubber impression that will be made.
The methyl methacrylate custom tray is first coated with a primer. Then a silicone of pasty consistency is mixed with a curing agent and some suitable catalyst, placed into the tray, and the tray is inserted into the patient's mouth where curing takes place. After this initial impression has cured, it is removed and trimmed to eliminate undercuts and thin-walled sections. Then a silicone compound of lower viscosity is catalyzed and inserted into the patient's mouth directly over the area to be repaired. The custom tray containing the cured silicone rubber initial impression is pressed firmly into place over the catalyzed silicone until curing is complete. This part of the procedure is then finished, and the now highly detailed impression of the patient's mouth is forwarded to a laboratory for the making of a bridge.
For the preparation of full dentures, the silicones are utilized in much the same manner, except that a re-usable metal perforated tray is used instead of a custom tray made of polymeric materials. The high viscosity silicone compound is placed into the tray as before, inserted into the mouth to make a first impression, and this is followed by the use of a lower viscosity silicone compound to make a more detailed impression of the area of the mouth to be repaired, substantially as described above.
An elastic silicone composition which can be used to obtain room temperature vulcanizing dental impressions as described above is disclosed in Nitzsche et al, U.S. Pat. No. 3,127,363, the disclosure of which is incorporated herein by reference. Nitzsche et al disclose organopolysiloxane elastomeric compositions that are vulcanizable at room temperature and are taught as being useful for a variety of purposes, including the making of casts of the mouth and teeth. However, these compositions are not specifically adapted to some of the requirements relating uniquely to the making of dental impressions.
Lampe, U.S. Pat. No. 3,696,090, incorporated herein by reference, discloses a room temperature curable silicone rubber composition which can be used making ear plugs. However, relatively small amounts of filler (10-50%) must be employed or the composition is difficult to mold.
There have now been discovered new improved two-part silicone rubber compositions which are vulcanized at room temperature and are adapted for making dental impressions. The present compositions provide desired properties such as moderately fast "pot life", putty-like consistency, good shelf stability, low linear shrinkage, and low-toxicity. Vulcanization of the compositions of the present invention is not inhibited by materials from which the custom trays are made, e.g., polyacrylics, polyethylene, polyvinylchlorides, epoxies, and the like.
The compositions achieve greater hardness (durometer build-up) within a shorter time after cure, and can be formulated to cure to a smooth, non-grainy appearance.
In addition, the need for custom tray fabrication by the dentist is eliminated because the compositions of the present invention can be employed both as the custom tray material and as the high viscosity compound which is inserted into the tray to make dental impressions. Moreover, bite registration, which usually requires a separate operation with either a custom tray made of methyl methacrylate or the like or a metal perforated tray, can now be obtained with the present silicone putty tray material at the same time as the dental impression.
The detailed compositions are not disclosed in the prior art, and the remarkable properties, as dental impression materials, are not obvious from anything disclosed or remotely suggested in the prior art.